1. Field of the Invention
The present invention relates to a self-calibration method for an electric power steering system, particularly to an EPS self-calibration method normalizing signals of sensors.
2. Description of the Related Art
Before, a powerful and smooth-operation HPS (Hydraulic Power Steering) system was used as an auxiliary power source to lessen the driver's burden of operating the steering wheel. However, hydraulic fluid is likely to leak from hydraulic piping. Further, hydraulic fluid is likely to heat up and denature because of friction between hydraulic fluid and piping. Besides, hydraulic piping is usually very complicated. Thus, the EPS (Electric Power Steering) system has gradually replaced the conventional HPS system.
Compared with the HPS system using a hydraulic pump, an oil tank, hydraulic piping and a belt-pulley system that transmits engine power to drive the pump, the EPS system is implemented by electronic signals and thus has lower fabrication cost and maintenance cost. Further, the EPS system provides different amounts of power for different driving conditions. The EPS system does not output power unless the condition needs it. The EPS system makes the driver to steer the vehicle more easily and accelerates the response of the vehicle. Therefore, the EPS system will be a standard apparatus of vehicles soon or later.
The EPS system uses a torque sensor and a current sensor. The steering sense of the driver is greatly influenced by signals of sensors of the EPS system, such as the torque sensor and the current sensor. Therefore, the sensors of EPS systems should be calibrated to a normal state before delivery. Because of long-term use or displacement of installation positions, sensors will drift from their normal states and generates signals having slight errors. The errors are measured and compensated for to restore sensors to have the original setting or better setting. Environmental factors, such as temperature, or instable power supply, may cause erroneous auxiliary torque or discontinuous auxiliary force and affect the performance of the EPS system. In assembling an EPS system to a vehicle, sensor normalization takes time. In mass production, the total time and manpower spent in normalizing the sensors of all the vehicles is very considerable. Therefore, how to overcome sensor accuracy degradation caused by power supply variation and promote robustness of EPS systems is a problem the manufacturers desire to solve.
Accordingly, the present invention proposes a self-calibration method for an electric power steering system to overcome the abovementioned problems.